Wenbin Mao

Anisotropic Micro‐ and Nano‐Capsules

In this work, we introduce anisotropically shaped, ultrathin micro- and nano-capsules fabricated by layer-by-layer approach. The original cubic and tetrahedral shapes of the template particles were replicated to produce hollow capsules with well-defined edges. Introducing tannic acid as a component of LbL shells resulted in enhanced chemical stability of these hollow polymer structures under a wide pH range due to high pK(a) value. Computational studies demonstrated increased mechanical stability of the anisotropic capsules under osmotic pressure variation due to sharp edges and vertices acting as a reinforcing frame in contrast to spherical microcapsules that undergo random buckling.

Mesoscale modeling: solving complex flows in biology and biotechnology

Fluids are involved in practically all physiological activities of living organisms. However, biological and biorelated flows are hard to analyze due to the inherent combination of interdependent effects and processes that occur on a multitude of spatial and temporal scales. Recent advances in mesoscale simulations enable researchers to tackle problems that are central for the understanding of such flows. Furthermore, computational modeling effectively facilitates the development of novel therapeutic approaches. Among other methods, dissipative particle dynamics and the lattice Boltzmann method have become increasingly popular during recent years due to their ability to solve a large variety of problems. In this review, we discuss recent applications of these mesoscale methods to several fluid-related problems in medicine, bioengineering, and biotechnology.

Inertial migration of deformable capsules in channel flow

Using three-dimensional computer simulations, we study the cross-stream inertial migration of neutrally buoyant deformable particles in a pressure-driven channel flow. The particles are modeled as elastic shells filled with a viscous fluid. We show that the particles equilibrate in a channel flow at off-center positions that depend on particle size, shell compliance, and the viscosity of encapsulated fluid. These equilibrium positions, however, are practically independent of the magnitude of channel Reynolds number in the range between 1 and 100. The results of our studies can be useful for sorting, focusing, and separation of micrometer-sized synthetic particles and biological cells.

Hydrodynamic sorting of microparticles by size in ridged microchannels

Sorting and separation are key elements in many microfluidic processes. Here, we use computational modeling to design a hydrodynamic method for high-throughput separation of solid microparticles by size in microchannels. The rapid and high-resolution separation occurs due to a combination of two hydrodynamic effects: cross-stream inertial migration of particles and circulatory fluid flows created by periodic diagonal ridges protruding from opposite channel walls. This new continuous separation method operates in a wide range of Reynolds numbers, is insensitive to the magnitude of channel flow rate, and features simple design that can be readily integrated into microfluidic devices for massive sample analysis.

Eyelashes divert airflow to protect the eye

Eyelashes are ubiquitous, although their function has long remained a mystery. In this study, we elucidate the aerodynamic benefits of eyelashes. Through anatomical measurements, we find that 22 species of mammals possess eyelashes of a length one-third the eye width. Wind tunnel experiments confirm that this optimal eyelash length reduces both deposition of airborne particles and evaporation of the tear film by a factor of two. Using scaling theory, we find this optimum arises because of the incoming flows interactions with both the eye and eyelashes. Short eyelashes create a stagnation zone above the ocular surface that thickens the boundary layer, causing shear stress to decrease with increasing eyelash length. Long eyelashes channel flow towards the ocular surface, causing shear stress to increase with increasing eyelash length. These competing effects result in a minimum shear stress for intermediate eyelash lengths. This design may be employed in creating eyelash-inspired protection for optical sensors.

Fluid transport in thin liquid films using traveling thermal waves

Using long wave theory and direct numerical solutions of the Navier–Stokes equations, we investigate thermocapillary flows arising in a thin liquid film covering a heated solid substrate with non-uniform temperature in the form of traveling thermal waves. Our results indicate that unidirectionally propagating interfacial waves are formed in the liquid film. The interfacial waves transport liquid, thereby creating a net pumping effect. We show that the frequency of thermal waves leading to the most efficient pumping is defined by their wave length and weakly depends on other system parameters. The results are useful for designing new methods for transporting liquids in open microfluidic devices.

Creating localized-droplet train by traveling thermal waves

We investigate the nonlinear dynamics of a two-layer system consisting of a thin liquid film and an overlying gas layer driven by the Marangoni instability induced by thermal waves propagating along the solid substrate. In the case of a stationary thermal wave with sufficiently large amplitude and Marangoni number, liquid film rupture takes place with a flattish wide trough. For sufficiently small but not too small frequencies of the thermal wave, a periodic structure consisting of localized drops interconnected by thin liquid bridges emerges. This train of drops travels unidirectionally along the heated substrate following the thermal wave. For larger thermal wave frequencies, the thickness of the bridges increases enabling fluid flow between the neighboring drops. The drop-train regimes may be utilized in microfluidic applications for directed transport of liquid content enclosed in drops formed by thermocapillary forces.

Magnetic microbeads for sampling and mixing in a microchannel

Microfluidics provides exciting possibilities for miniaturized biosensors systems allowing for highly parallel automated high throughput tests to be performed. Detection of low concentrations of bacteria, viral particles and parasites in food samples is a challenging process. The capture of the target can be more effectively carried out with efficient mixing. We present a simple microfluidic system capable of controlled transport of rotating magnetic beads among soft magnetic patterns. Low aspect ratio NiFe discs (200 nm tall, diameter 3 μm) are patterned onto a silicon wafer. A PDMS channel is bonded onto the wafer to create the microfluidic channel. An external permanent magnet attached to a motor provides a magnetic field, which can be rotated at different speeds while magnetizing the NiFe disks in the channel. Microbeads (Dynabeads M-280, Invitrogen) introduced into the channel with a syringe pump are trapped at the poles of the now magnetized soft magnetic discs. Rotation of the external permanent magnet induced magnetic poles in the soft magnetic discs which will in turn rotate the trapped microbeads. We have already demonstrated the capacity to capture particles from flow with rotating M-280 beads in this device.

Stiffness Dependent Separation of Cells in a Microfluidic Device

Rapidly sorting and separating cells are critical for detecting diseases such as cancers and infections and can enable a great number of applications in bio-related science and technology. While a variety of techniques demonstrate separation by physical parameters such as size[1] and mass[2], inexpensive and easy to use methods are needed to separate cells by mechanical compliance. A number of pathophysiological states of individual cells result in drastic changes in stiffness in comparison with healthy counterparts. Mechanical stiffness has been utilized to identify abnormal cell populations in detecting cancer[3–5] and identifying infectious disease[4, 6]. Recently, microfluidic methods were developed to classify and enrich cell populations utilizing mechanical stiffness[7–9]. We demonstrate a new strategy to continuously and non-destructively separate cells into subpopulations of soft and stiff cells. In our microfluidic separation method, we employ a microchannel with the top wall decorated by a periodic array of rigid diagonal ridges (Fig. 1). The gap between the ridges and the bottom channel wall is smaller than the cell diameter, thus the cells are periodically compressed by the ridges. The difference in mechanical resistance to compression of cells gives rise to a stiffness-dependent force associated with cell passage through narrow constrictions formed by the consecutive channel ridges. This elastic force is directed normal to the compressive diagonal ridges and, therefore, deflects cells propelled by the flow in the lateral direction with a rate proportional to their compliance. In this paper, we employ this principle to separate modified lymphoblastic cells with dissimilar mechanical stiffness in high-throughput.Copyright